High-Resolution TEM Observation of Sn Nanoparticles on SnO2 Nanotubes 1113
structure, and will not form an interface with SnO2. In this study, the agglomeration of individual Sn NPs into larger Sn NPs was observed while the interface between Sn and SnO2 was also maintained, suggesting that the aforementioned energies were similar in magnitude and took a part simul- taneously in the reaction process. To confirm the reliability of the in situ TEM observa-
Figure 12. a: Charge and discharge profile of SnO2 nanotubes (NTs) in the formation cycle showing the respective arrows (red, blue, and green) that correspond to each stage. b: Ex situ transmis- sion electron microscopy (TEM) images of Sn nanoparticles (NPs) formed at each stage (red, blue, and green). c: The calculated aver- age size of Sn NPs in each stage (red, blue, and green). d: Ex situ high resolution-TEM image and corresponding FFT pattern of Sn NP.
tions that we undertook using GLC, ex situ TEM images of SnO2 NTs after electrochemical lithiation were taken for comparison. Electrodes consisting mainly of SnO2 NTs were slurry cast and assembled into half-type 2032 coin cells for electrochemical cell tests. Figure 12a shows the charge and discharge profile of SnO2 NTs during the cell test. It can be seen that the plateau region initiated at about 0.95V, indicating the point at which the conversion reaction started to take place. Initially, the SEI layer (which was confirmed as LiF according to EELS spectra) was formed on SnO2 NTs (Fig. 13), which allows suitable ionic transport (Cheong et al., 2016). Based on the initial voltage profiling, the similar coin cell was run and taken out at 0.95V (red), 0.9V (blue), and 0.85V (green) for ex situ characterization. The resulting TEM images of SnO2 NTs at three different stages of lithiation (marked by a red, blue, and green arrow in the charge–discharge profile) were taken and shown in Figure 12b, where the sizes of Sn NPs increased progres- sively. For further analysis, we have taken TEM images of electrode materials in 25 different locations at each stage (red, blue, and green) and further calculated the average sizes of Sn NPs. They were plotted in the graph as shown in Figure 12c. Similar to patterns shown in Figure 7d, the average size of Sn NPs significantly increased from 10.5 to 27nm, which is suggests the agglomeration of Sn NPs. HR- TEM image and FFT patterns of the NPs were taken (Fig. 12d) to further confirm that they are β-Sn. Based on above results, it is clear that both in situ and ex situ TEMimages are generally in good agreement.
CONCLUSION
Figure 13. a: Ex situ transmission electron microscopy (TEM) image of SnO2 nanotubes (NT) showing the formation of solid electrolyte interface (SEI) layer in the initial conversion reaction. b: Corresponding electron energy loss spectroscopy (EELS) spectrum of SEI layer.
Sn NPs, which in turn minimizes the surface energy (Zhang et al., 2012). During the dynamic agglomeration process, both the surface and interfacial energies balanced each other and determined the general morphology of the Sn NPs. If creating an interface between Sn and SnO2 is more stable than elemental Sn existing as spherical NPs (i.e., γi<γs), the Sn particles will disappear to be replaced with a uniform Sn layer, which maximizes the interfacial area between Sn and SnO2. However, if γi>γs, Sn NPs will maintain their spherical
In conclusion, using GLC-TEM, we successfully visualized the growth process of Sn NPs on SnO2 NTs in a liquid electrolyte. This process is an intermediate step between the SEI layer formation by chemical lithiation and the full conversion reaction. It is demonstrated that Sn NPs merge forming the interface with SnO2 surface, affected by the two driving forces: interfacial energy between SnO2 and Sn and surface energy of Sn. This work sheds light on the in situ study of morphology and phase transitions at interface between the electrolyte and electrode materials, which are important in optimization of nanostructured electrodes.
Supplementary Material
Supplementary Material can be found online. Please visit
journals.cambridge.org/jid_MAM.
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